1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to an automatic brightness stabilization (ABS) apparatus that minimizes color shifts caused by degradation of an electron gun in an image display device that adopts a cathode ray tube (CRT), and more particularly, to an ABS apparatus that prevents brightness stabilization from being affected by a dynamic focus device and a velocity modulation device.
2. Description of the Related Art
An ABS circuit is designed to minimize color shifts caused by degradation in an electron gun. A conventional ABS circuit inserts a pulse signal of a given level into a flyback period of an image signal applied to the electron gun of a CRT. By comparing a level of a pulse signal affected by the operation of the electron gun with a normal level of the pulse signal (when the electron gun is not degraded), the conventional ABS circuit determines the degree of degradation in the electron gun and then changes the amplification degree of the image signal applied to the electron gun according to the determined degree of degradation in the electron gun. In this way, the conventional ABS circuit minimizes color shifts even when the electron gun is degraded.
In image display devices, a velocity modulation circuit for compensating for the edges of an image signal is used along with a dynamic focus circuit for compensating for focus shifts in and around the middle of a screen.
At the edges of a luminance signal, the velocity modulation circuit applies a velocity modulation signal to a velocity modulation coil mounted in the CRT and controls a scanning velocity of beam generated in the electron gun, thus emphasizing the edges of the image signal. The dynamic focus circuit applies a horizontal parabola signal to a horizontal deflection signal and applies a vertical parabola signal to a vertical deflection signal, thereby compensating for focus shifts in and around the middle of the screen.
In a typical CRT, a dynamic focus signal and the velocity modulation signal have voltages of at least 100V while the image signal applied to the electron gun has a voltage of at most several volts.
As a result, noise components caused by the dynamic focus signal and by the velocity modulation signal are mixed with the pulse signal detected for automatic brightness stabilization, causing instability of brightness stabilization.